To remedy this drawback, a U-shaped receiving head has been designed having a spherically symmetrical outside surface and into which a threaded connecting rod is inserted so that two nuts can be screwed to opposite sides of the head, forming a vice that holds the pedicular screw and the connecting rod in a fixed position. Because the screwhead is spherically symmetrical, the rod can be oriented in different positions relative to the screw; its positions relative to the screw are restricted to the plane of symmetry between the two branches of the U-shaped screwhead and containing the body of the screw.
Furthermore, the two nuts forming the vice also have the clamping force drawback, because the nuts are screwed directly against the receiving head.
An object of the present invention is to remedy those drawbacks by proposing a fixing device with which the clamping force is increased.
This object is achieved by virtue of the fact that the device includes a first longitudinal member having a first surface and an opposite second surface, a first end of said first surface being adapted to be pressed against a portion of a first hemispherical part of said screwhead and a middle part of said second surface being adapted to bear against a portion of a first part of said rod, a second longitudinal member adapted to co-operate with said first longitudinal member, whose first end is adapted to be pressed, opposite a portion of said first end of said first surface of said first member, against a portion of a second hemispherical part of said screwhead opposite the first hemispherical part and whose middle part is adapted to bear against a portion of a second part of said rod opposite the first, and first fastening and adjustment means adapted to hold the second ends of said first and second longitudinal members facing each other and to move them toward each other so as to cause said second longitudinal member to pivot around said rod relative to said first longitudinal member to grip said screwhead between said first end of said first surface and simultaneously to grip said first end of said second longitudinal member and said rod between said middle part of said second surface of said first longitudinal member and the middle part of said second longitudinal member, whereby said device connects said rod and said screwhead and holds them in a fixed position relative to each other.
Thus one feature of the fixing device is the method of immobilizing and clamping the receiving head and the rod by pivoting two longitudinal members relative to each other about the rod so that the first ends of said longitudinal members form a first clamp adapted to grip the spherically symmetrical screwhead and their middle parts form a second clamp adapted to grip the rod simultaneously. The two longitudinal members pivot about the rod by virtue of the first fastening and adjustment means, which enable the second ends of said longitudinal members to be moved forcibly toward each other. In this way, through a lever effect, the force applied to move the second ends toward each other enables the first ends of the longitudinal members to form a vice and to grip the screwhead and the rod simultaneously with a force much greater than that obtained by clamping an immobilizing part directly against the rod.
Furthermore, because the rod is gripped near the spherically symmetrical screwhead, the screwhead has no notch, into which the rod would otherwise have to be inserted, and therefore forms a true ball-and-socket joint, so that the positions of the rod relative to the pedicular screw are no longer limited to one plane.
In a preferred embodiment, the first end of said first surface forms a spherical socket whose bottom includes a bore opening onto the end of the second surface of said longitudinal member, said spherical socket being adapted to be pressed onto said screwhead. Thus the first end is locked perfectly against the screwhead because the end of the surface mates perfectly with the surface of the screwhead. Furthermore, the bore that is formed in the part at the bottom of the spherical socket provides access to the top of the screwhead, which has a hexagonal housing for screwing in the pedicular screw. Thus the screw can be screwed tightly into the vertebra even if the fixing device is already mounted on the screwhead.
The middle part of said second surface advantageously includes a first housing in which said portion of a first part of said rod is adapted to bear. Thus the rod is prevented from moving in translation in the direction of the first longitudinal member, which improves the immobilization of the rod relative to the head.
In one particularly advantageous embodiment, the second longitudinal member has a portion facing said second surface of said first longitudinal member and a curved portion forming said first end of the said second longitudinal member. The second longitudinal member, bearing on the rod, is therefore able to pivot around the rod within the limits of contact with the first longitudinal member. When the first longitudinal member is bearing against a hemispherical first part of the screwhead, the curved portion of said longitudinal member is pressed against the second hemispherical portion of the screwhead to grip it.
The first end of said second longitudinal member advantageously has a truncated spherical socket portion adapted to be pressed against said portion of the second hemispherical part of said screwhead. Accordingly, the shape of the end of said second longitudinal member coincides perfectly with the surface of the screwhead, which improves fastening.
Said portion facing said second surface of said first longitudinal member preferably has a bore opening opposite said bore opening onto the end of the second surface of said longitudinal member, through which said screwhead is accessible when said device is mounted.
In one particular embodiment, the middle part of said second longitudinal member includes a second housing adapted to receive said rod and situated opposite said first housing. Thus said rod is also immobilized against movement in translation parallel to said second longitudinal member. The rod can therefore be gripped between the two longitudinal members.
The first fastening and adjustment means advantageously include a screw adapted to pass through said second ends of said first and second longitudinal members, whose head is pressed against said end of said second longitudinal member and whose body is screwed into an internal screwthread in the second end of said first longitudinal member. Accordingly, tightening the screw tends to move the second ends of the longitudinal members toward each other and to hold them together, their middle portions respectively bearing, in order to form pivots, against a portion of a first part of said rod and against a portion of a second part of said rod opposite the first part, so that the first ends of said longitudinal members grip the screwhead to immobilize the assembly.
In a first particular embodiment, the invention is applied to a spine stabilizing system including two spherical head pedicular screws each adapted to be screwed into a pedicle of a vertebra, two fixing devices according to the invention mounted on said screwheads with said longitudinal members substantially parallel to each other and to the axis of the spine, a transverse rod substantially perpendicular to the axis of the spine adapted to join said two devices together, and a connecting system adapted to connect said transverse rod to a longitudinal rod substantially parallel to the axis of the spine.
Thus this first particular embodiment of the stabilizing system holds the vertebrae of a portion of the spine together when the anatomical members adapted to provide this function are missing. To this end, the fixing devices according to the invention mounted on each of the two pedicular screws fixed into a vertebra are joined together by a transverse rod forming a rung on the vertebra, and the rungs are joined together by longitudinal rods by means of a connecting system adapted to fasten together two substantially perpendicular rods.
In a second particular embodiment, the invention is applied to a spine stabilizing system including two spherical head pedicular screws adapted to be screwed into the pedicles of two superposed vertebrae, two fixing devices according to the invention mounted on said two screwheads with said longitudinal members substantially parallel to each other and perpendicular to the axis of the spine, and a longitudinal rod substantially parallel to the axis of the spine adapted to connect said devices together.
Accordingly, the devices according to the invention are mounted on two pedicular screws fixed into two successive vertebrae, on the same side of the spine, and are joined together by a rod so as to hold the two facing pedicles of two successive vertebrae in a fixed position. To reinforce the spinal column symmetrically, a similar system is fitted on the other side of the spine.
In a third particular embodiment, the invention is applied to a spine stabilizing system including at least two spherical head pedicular screws each adapted to be screwed into a pedicle of a vertebra, two devices according to the invention mounted on said screwheads with said second ends of said longitudinal members directed towards each other, and a transverse connecting part fastened to a second end of said parts of said two devices and adapted to connect said two devices together.
Just as in the preceding particular embodiment, the fixing device according to the invention connects superposed pedicular screws with rods disposed parallel to the axis of the spine. However, in this third embodiment, the devices are connected together, transversely from one side of the spine to the other, by a transverse connecting part.
In a fourth particular embodiment of the invention, the fixing device further includes second adjustable fastening means fastened to said second longitudinal member and situated between its first end and its middle part, and said second fastening means are adapted to bear against the first end of said second surface of said first longitudinal member so as to move said first end of said second longitudinal member and said portion of said first end of said first surface of said first member toward each other to trap said screwhead, whereby said longitudinal members are adapted to pivot relative to each other about said spherically symmetrical screwhead between a position in which said second ends of said longitudinal members are spaced apart from each other and a position in which said second ends of said longitudinal members are close to each other.
Accordingly, apart from the fact that it improves the immobilization of the spherically symmetrical screwhead, this feature also fastens the first ends of the longitudinal members together in a non-demountable fashion, without said second ends of said first and second longitudinal members having to be connected by said first fastening and adjustment means. The rod therefore be adjusted relative to the two longitudinal members, which remain fastened to the spherical symmetrical screwhead before they are immobilized by moving said second ends of said second longitudinal members toward each other by means of the first fastening means.
Said second fastening means advantageously consist of a screwthreaded member adapted to co-operate with an internal screwthread formed in the wall of said first bore, said screwthreaded member being adapted to bear against the first end of said second surface of said first longitudinal member. Accordingly, rotation of said screwthreaded member entrains it in translation in the direction of the first end of said second surface of said first part, or in the opposite direction.
Said second end of said second longitudinal member preferably has a U-shaped recess opening in the direction away from said first end, said screw passing in an inclined manner through said second end of said first longitudinal member so that said screwhead is spaced apart from said first end when it is unscrewed and close to said first end when it is screwed in, whereby said screw is adapted to be engaged in said U-shaped recess, with said screwhead bearing against the perimeter of said recess.
Thus, by virtue of this feature, the second longitudinal member can be pivoted around the screwhead, moving said second ends away from each other, when said second adjustable fastening means are operated, with the screw remaining in an unscrewed position in said first longitudinal member, in which unscrewed position the screwhead is remote from the first end. Thus the screw can be inserted before mounting the device and then tightened to immobilize said rod, as explained in more detail later in the description.
It is particularly advantageous if said first longitudinal member further includes a second bore situated between said middle part and second end of said first longitudinal member, opening onto said first and second surfaces, respectively, and adapted to receive a screw whose head is pressed against said second surface to hold said first longitudinal member in a fixed position relative to a wall with said first surface against said wall. In the fourth embodiment of the invention, this particular feature enables said fixing device to be connected to the lateral walls of the vertebral bodies.
The fourth embodiment of a device according to the invention is advantageously applied to a spine stabilizing system which includes two fixing devices according to the invention and two spherical head posterior screws each adapted to be screwed into the posterior lateral wall of the vertebral body of a respective one of two superposed vertebrae, in which system said first longitudinal members are adapted to be held in a fixed position relative to each other, substantially parallel to each other and to the median planes of said vertebrae, and against the lateral wall of the vertebrae by anterior screws passing through said second bore, the first ends of the first surfaces respectively bearing against said spherical heads, and the two fixing devices being held in a fixed position relative to each other by a single rod gripped by the median parts of respective longitudinal members. Thus the two vertebrae are held relative to each other, so that in particular said assembly partly supplants the intervertebral disc.